TRISO NUCLEAR FUEL ARTICLE ►To Home Page ● Chapter Three Index
Overview: Better Than "Clean Coal": TRISO, the Alternative Nuclear Fuel
Worldwide, there are 141,000 fossil fuel power plants making 70% of the accumulating CO2 that is causing Global Warming. Eventually these power plants will have to have their boilers converted from coal to CO2-free nuclear in mankind’s biggest CO2 mitigation project.
These 141,000 power plants will always overwhelm all other efforts to end Global Warming. They must be stopped. Unavoidable if we wish to stop Climate Change, this technically and economically frugal conversion from coal to nuclear is only possible because of TRISO nuclear fuels.
Recycling the world’s coal-burning power plants will make CO2 mitigation much more economically attractive for the less wealthy second and third world countries since it will enable them to continue to use their existing electrical grids, electrical substations, cooling water systems, turbine-generators, railroads, access roads, and their already trained, highly skilled workers.
About TRISO Nuclear Fuels
Conventional water-cooled reactors can produce steam no
hotter than 600°F - far short of the 1,000°F all fossil fuel power plants need.
The only nuclear reactors that can produce heat hot enough to replace coal’s
2,000°F fire are TRISO-fueled High-Temperature Gas-cooled Reactors (HTGRs).
HTGRs are much less expensive, mass-producible, largely automatic
“mini-reactors” about 1/5 as powerful as today’s huge conventional nuclear reactors.
TRISO fuels can withstand temperatures as high as 3,600°F but TRISO nuclear fuel heated reactors typically “cruise” at 1,700°F and its nuclear fuel is usually formulated to cause a natural physics phenomenon called Doppler-broadening to keep the reactors from ever getting hotter than about 2,700°F, leaving an unalterable natural safety margin of almost 1,000°F, thus making the reactor "walk-away" safe.
TRISO nuclear fuels are called "TRI-SO" for the tristructural- isotropic (triple carbon layer coated) poppy-seed size micro-particles of nuclear fuels (TRISO can be blends of uranium, thorium, and plutonium) that make up their bulk. This multi-coating containment of the nuclear fuel particles with layers of silicon carbide and pyrolytic carbon is necessary to keep the fission waste product gas, xenon-135, which can stop the reactor, from getting loose in the reactor. Additional containment also means additional safety.
Further enhancing a pebble bed reactor’s overall safety, these triple coatings also provide three additional levels of containment of the radioactive fuels in addition to the two containment barriers created by the reactor itself and its underground silo. Conventional liquid cooled nuclear power reactors provide only one or two levels of containment and also cause large amounts of cooling water to come into direct contact with the uranium - a radiation hazard. The typical TRISO reactor's gas coolant, helium, does not become radioactive.
Producing pebbles.
To produce an 8 ounce nuclear pebble containing about 9 grams of nuclear fuel,
some 15,000 of these tiny triple-coated TRISO fuel particles are mixed with a
graphite powder/phenolic resin paste (the speed moderator used to slow the
neutrons down) and pressed into the shape of a 50 mm
diameter ball. A 5 mm thick outer shell of pure carbon is then added and the
pebble is sintered, annealed, and machined smooth to 60 mm to withstand
the abrasion, impacts, and heat of automated machine handling and fluidized
travel through the reactor’s pebble bed.
When fresh, a TRISO pebble
can produce more than 1,000 watts of heat. It has a
A TRISO reactor can contain up to 450,000 pebbles or prisms and will consume on average about 400 pebbles per day while running at full power. Since the pebbles in pebble bed reactors are being constantly circulated, measured for their remaining power, removed when "tired," and replaced with a fresh pebble, there is no month-long refueling shutdown as is the case for conventional power plant reactors. "Waste" TRISO pebbles must be crushed into a powder before they can be recycled to recover the 95% of their energy that remains. It will take 10 to 15 recycle cycles before the waste is reduced to nothingness.
TRISO-fueled reactors are simple silos containing either a bed of pebbles or racks of prisms, usually being cooled by circulating helium gas, which in turn is cooled by the power plant’s steam boiler. The South African (PBMR) pebble bed’s full output is165 megawatts electrical, getting 90 megawatt-days of electricity per kilogram “mileage” from its 9% enriched uranium pebbles.
TRISO tomorrow
The multi-national Generation-IV reactors program is developing a higher temperature (5,000°F ?) TRISO reactor capable of economically splitting water directly into its hydrogen and oxygen atoms without producing the 5 pounds of CO2 each pound of today's hydrogen causes. Massive amounts of hydrogen are necessary to upgrade poor quality fossil fuel hydrocarbons like oil sands, oil sludge, and oil shale into higher-quality crude oils refinable into gasoline. Having massive amounts of hydrogen and heat available also make conversion of algae into carbon-neutral gasoline, diesel, and jet fuel practical.
PBMR, Pty., has a pebble bed reactor facility under construction at Koeberg, South Africa. China has a teaching pebble bed reactor at Tsinghua University that has run since 2000 and China will begin building a 19-pebble bed reactor facility at Rongcheng in 2009. As many as a thousand Chinese HTR-PM 100 MWe pebble bed reactor steam plants are planned as electricity and heat sources for China’s remote cities in addition to China’s planned 100 large 1,600 MWe PWR reactors.
© James P Holm, 2008
One of the unique features of the high temperature gas-cooled reactor is the TRISO fuel used for the fission reaction. All high temperature gas-cooled reactors use small (~1mm diameter) fuel particles to accomplish the nuclear reaction. These small fuel particles have a kernel of enriched uranium in the form of an oxide (or oxycarbide). The kernel is subsequently coated with a porous carbon layer (to absorb fission gases), a dense pyrolytic carbon layer, a silicon carbide layer and finally another pyrolytic carbon layer.
The coatings surrounding the kernel of
TRISO particles produce a very robust fuel form by acting as the
containment boundary for the radioactive material. These coatings work
in much the same way as the massive reinforced concrete structure
surrounding the light water reactors currently in service. The TRISO
coatings confine the fission products within the fuel particle.
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The Next Generation Nuclear Plant (NGNP) project, Copyright
© 2008 Idaho National Laboratory